1. Field of the Invention
The present invention relates to an improvement in an apparatus for controlling the amount of ink (hereinafter referred to an ink-amount control apparatus) in a printing press, and particularly to an ink-amount control apparatus that is capable of properly controlling the amount of ink even in a case where an image to be printed is small, an image is printed only on the part of a surface of substrate or an image to be printed lies only on the part of a printing plate.
2. Related Art
A printing press of the above type is provided with an ink unit for each plate cylinder for supplying ink to an printing plate of the plate cylinder. The ink unit supplies ink reserved in an ink fountain to the printing plate via plural ink rollers. The ink fountain is made up of an ink fountain roller and a blade. Ink flowing onto the outer circumference of the ink fountain roller via a gap between the ink fountain roller and the blade is transferred to the ink rollers located downstream to the ink fountain via an ink transfer roller. Like a pendulum, the ink transfer roller moves back and forth between the ink fountain roller and a most upstream one of the ink rollers with respect to an ink transfer direction. The number of times at which the ink transfer roller touches the ink fountain roller, per a given number of rotations of the plate cylinder remains constant throughout the printing operation, so that the more frequently the ink transfer roller touches, the larger the amount of ink is transferred to the ink roller.
The blade is made up of plural ink fountain keys aligned parallel to the axis of the ink fountain roller (a widthwise direction of a substrate fed in the printing press), with the opening degrees of the ink fountain keys being adjustable independently of each other. The larger the gap relative to the ink fountain roller or the larger the opening degree of each ink fountain key, the larger the amount of ink flowing onto the outer circumference of ink fountain roller. In recent years, an operation is made such that the image area ratio of a printing plate is calculated for each zone thereof corresponding to each ink fountain key and then the image area ratio for each zone is converted into the opening degree of each ink fountain key based on a given conversion curve, thereby presetting the thus converted opening degree of each ink fountain key prior to the printing operation. According to this presetting of the opening degree of each ink fountain key, where the image area ratio is large in a certain zone, the opening volume of a corresponding ink fountain key is increased, on the other hand, where the image area ratio is small in a certain zone, the opening degree of a corresponding ink fountain key is decreased so as to allow the decreased amount of ink flow onto the ink fountain roller for supply of a proper amount of ink. This operation may however cause a problem that the relationship between the opening volume of each ink fountain key and the amount of ink adhered to the outer circumference of the ink fountain roller is unlikely to be kept proportional after the opening degree of each ink fountain key has been decreased to a certain degree. That is, the amount of ink adhered to the ink fountain roller is not decreased corresponding to a decreased image area ratio even by setting a corresponding ink fountain key to a smaller opening degree. This results from various reasons such as leakage of ink through a gap, which may be caused in actual operation even after setting each ink fountain key at 0, or ink flow onto an adjacent portion due to the reciprocal motion of an oscillating roller in a lateral direction of the printing press, that is, a direction parallel to the axis of each roller. Thus, a larger amount of ink than an ideal amount tends to be supplied to the printing plate when the image area ratio is small.
Where an image lies only on the part of a substrate, many zones without images exist, which necessarily causes many zones with remarkably small image area ratios. This causes a problem of making it hard to perform color matching process for a print with an image printed only on the part thereof.
In light of the above problem, it is an object of the present invention to provide an ink-amount control apparatus in a printing press that is capable of providing for excellent color and detail reproduction of an image with high precision as compared with a conventional apparatus, even if the mage lies only on the part of a substrate.